In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. For example, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements (lithographic printing plate precursors) useful to prepare lithographic printing plates typically comprise one or more imageable layers applied over the hydrophilic surface of a substrate. The imageable layers include one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the imageable element is considered as positive-working. Conversely, if the non-imaged regions are removed, the imageable element is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital or thermal imaging has become increasingly important in the printing industry because of their stability to ambient light. The lithographic printing plate precursors used for the preparation of lithographic printing plates have been designed to be sensitive to heat or infrared radiation and can be exposed using thermal heads or more usually, infrared laser diodes that image in response to signals from a digital image in a computer a platesetter. This “computer-to-plate” technology has generally replaced the former technology where masking films were used to image the elements.
These imaging techniques often require the use of water or a developer (neutral to alkaline pH) as a processing solution to remove exposed (positive-working) or non-exposed (negative-working) regions of the imaged layer(s). In general, the processing solution is specifically designed for the specific radiation-sensitive chemistry in the imaged precursor even though there is a general desire in the industry to design one processing solution that can be used with much different imaging chemistry.
Contrast dyes are often incorporated in precursor imaging layer formulations in order to provide a color for easy precursor handling and enabling inspection of defects during manufacturing. The color from a contrast dye serves an important function in the pre-press readability of dots for calibrations and in the printing press room to recognize register marks for precise fitting on the printing press. In positive-working lithographic printing plate precursors, both single-layered and double-layered precursors, contrast dyes often serve the additional function of causing inhibition of the imaging formulation to developer attack and thereby stabilizing the dots in the resulting image. However, a significant problem has been encountered with the use of highly colored contrast dyes. They can cause “staining” of the non-imaged regions after development (processing).
One way to reduce this post-processing staining is to apply an interlayer on the aluminum substrate before the image layer formulation(s) are applied. An interlayer that is commonly used is poly(vinyl phosphonic acid) (PVPA) that can be applied either as a spray or in a coating bath process. For many processing methods that use low pH developers or high pH, silicate-containing developers, the presence of the PVPA interlayer is sufficient to avoid staining from the contrast dyes.
However, when high pH, non-silicate-containing developers are used to process some positive-working imaging formulations comprising a strong contrast dye, high staining can occur after development. One approach to solving this problem has been to use an alternative interlayer (instead of the PVPA interlayer). For example, some in the industry have incorporated an interlayer formed from a phosphate/fluoride (PF) process to reduce staining.
However, the use of this phosphate/fluoride process is undesirable because of the hazardous materials used or formed (HF and cryolith) in the process. In addition, considerable expense is required to install the equipment necessary to use the phosphate/fluoride process and such equipment requires additional space in the manufacturing facility, adding further costs.
There is a need to find useful contrast dyes for positive-working lithographic printing plate precursors that cause no staining when PVPA interlayers are used on the substrates and when the imaged precursors are processed using a high pH, non silicate developer.